default_read_to_end can't try_reserve probe reads

Author: kornelski

library/std/src/io/mod.rs

@@ -430,6 +430,8 @@ pub(crate) fn default_read_to_end<R: Read + ?Sized>(
         loop {
             match r.read(&mut probe) {
                 Ok(n) => {
+                    // there is no way to recover from allocation failure here
+                    // because the data has already been read.
                     buf.extend_from_slice(&probe[..n]);
                     return Ok(n);
                 }
@@ -826,15 +828,23 @@ pub trait Read {
     ///
     /// ```no_run
     /// # use std::io;
-    /// # struct Example; impl Example {
-    /// # fn read_some_data_for_the_example(&self) -> &'static [u8] { &[] }
-    /// fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
-    ///     let data_read = self.read_some_data_for_the_example();
-    ///
-    ///     buf.try_reserve(data_read.len()).map_err(|_| io::ErrorKind::OutOfMemory)?;
-    ///     buf.extend_from_slice(data_read);
-    ///
-    ///     Ok(data_read.len())
+    /// # struct Example { example_datasource: io::Empty } impl Example {
+    /// # fn get_some_data_for_the_example(&self) -> &'static [u8] { &[] }
+    /// fn read_to_end(&mut self, dest_vec: &mut Vec<u8>) -> io::Result<usize> {
+    ///     let initial_vec_len = dest_vec.len();
+    ///     loop {
+    ///         let src_buf = self.example_datasource.fill_buf()?;
+    ///         if src_buf.is_empty() {
+    ///             break;
+    ///         }
+    ///         dest_vec.try_reserve(src_buf.len()).map_err(|_| io::ErrorKind::OutOfMemory)?;
+    ///         dest_vec.extend_from_slice(src_buf);
+    ///
+    ///         // Any irreversible side effects should happen after `try_reserve` succeeds,
+    ///         // to avoid losing data on allocation error.
+    ///         self.example_datasource.consume(src_buf.len());
+    ///     }
+    ///     Ok(dest_vec.len() - initial_vec_len)
     /// }
     /// # }
     /// ```